1. Field of the Invention
The present invention relates to magnetite particles and more particularly it relates to magnetite particles which have the surfaces thereof covered with iron-zinc oxides, if necessary, further covered with iron-silicon oxides not only to assume a black color but also to make well-balanced improvements in magnetic properties and further make enhancement in dispersibility through an improvement of flowability which is otherwise poor due to agglomeration of the particles and are therefore useful especially as a powdery material for an electrostatic copying magnetic toner and as a black pigment powder for a paint, and also to a process for preparing the same.
2. Prior Art
These days, magnetic magnetite is used in a variety of fields, especially in a magnetic toner for electrostatic copiers. In general, processes for preparing such magnetic magnetite are classified into dry processes and wet processes. In the latter processes, prevalent processes are those wherein an aqueous solution of an alkali is added to an aqueous solution containing ferrous ions to effect an oxidation reaction under specific conditions.
In connection with these wet processes, for the purpose of improving various properties of powder such as magnetite particles, there have been proposed several kinds of magnetite particles containing zinc and processes for preparing the same. Typical of the proposed processes is a process as disclosed in Japanese Patent Publication Gazette No. 43,408/84. This process is one wherein spinel type iron oxide is obtained by adding an aqueous solution of an alkali to a ferrous salt, then effecting oxidation off the resulting mixture until a ratio of bivalent Fe to total Fe in the aqueous solution comes to fall within a specific range, subsequently adding a water-soluble zinc compound in such an amount as to provide a specific Zn/Fe ratio and then effecting a reaction at a specific free hydroxyl group ion concentration and at a specific solution temperature. The particles obtained according to this process are stated in said Gazette to have the features that "they assume a perfect (nonbrownish) black color and have a coercive force of about 100 to 200 Oe." Although these particles have reasonable properties, their saturation magnetization and the like are far from sufficient and they are very large in particle size, thus raising such a problem as to the quality of the resulting magnetic toner, especially as to the fact that the particles, particularly when dispersed in the toner, give thereto adverse effects such as excessive protrusion thereof from the surfaces of the toner and maldistribution thereof in the toner.
On the other hand, for the purpose of improving various properties of powder such as magnetite particles, there have also been proposed some kinds of magnetite particles containing a silicon component and processes for preparing the same, representative examples of which are magnetite particles and processes for preparing the same as disclosed in Japanese Patent Laid-Open Gazette No. 139,544/79, Japanese Patent Laid-Open Gazette No. 278,131/87, Japanese Patent Publication Gazette No. 9,045/91, etc.
Among the Gazettes so listed, Japanese Patent Laid-Open Gazette No. 139,544/79 discloses magnetic particles prepared by diluting fine particles of silicon oxide, a silicone oil and the like with an organic solvent to prepare a slurry and then effecting a treatment of bonding the original magnetic particles with the slurry. These particles involve a problem of uniformity of the silicon component on the particles because the treatment is a mechanical means.
On the other hand, Japanese Patent Laid-Open Gazette No. 278,131discloses magnetic iron oxide containing silicon element, characterized in that the proportion of the silicon clement present in the magnetic iron oxide is 0.1 to 1.5 wt. % based on the iron clement, and in that the silicon element content present in an eluted iron element of the magnetic iron oxide when a rate of iron element eluted therefrom reaches up to about 10 wt. % is at most about 0.7 wt. % and the silicon element content present in an eluted iron element of the magnetic iron oxide when a rate of iron clement eluted therefrom reaches between about 90 wt. % and 100 wt. % is about 0.2 to 5 wt. %, with a proviso that the ratio between the former and the latter is at least 1.0. Since the silicon content present in these particles between their surfaces and their 10% insides from their surfaces is only about 0.2% in terms of Si/Fe molar ratio when calculated based on the total amount of the particles, thus these particles still raising a problem of powder flowability.
On the other hand, Japanese Patent Publication No. 9,045/91 discloses particles prepared according to a procedure comprising preliminarily adding a water-soluble silicate in an amount of 0.1 to 5.0 atomic % in terms of Si based on Fe to either an aqueous solution of a ferrous salt containing a ferrous hydroxide colloid or an alkali hydroxide before the ferrous salt solution and the alkali hydroxide are reacted together. Since the silicate is added before the reaction, however, the resulting particles are notably low in silicon content in the proximity of the surfaces of the particles, this raising a problem of powder flowability.